The present invention relates to a drive unit for an alternator of a motor vehicle.
As is known, alternators of motor vehicles each comprise a shaft which rotates around its own axis, and is rotated by a belt drive unit, which comprises a drive pulley keyed onto the drive shaft of the motor vehicle, a driven pulley which is keyed onto the shaft of the alternator, and a drive belt which is wound around the drive and driven pulleys. It is apparent that the speed of rotation of the shaft of the alternator depends on the speed of the drive shaft, and on a pre-determined drive ratio, which is defined by the ratio between the diameters of the drive and driven pulleys.
As is known, an alternator distributes a current which is all the greater, the greater the angular speed of its own shaft. In particular, the characteristic of the current distributed by the alternator comprises a first section, in which the current distributed increases as the angular speed of the shaft of the alternator increases, and a second section, in which the increase in the current becomes virtually negligible when the angular speed varies. In addition, the energy dissipated by the alternator increases as the speed of rotation increases, in particular owing to the forces of friction, and to the actuation of a corresponding cooling fan.
In order to meet the requirements for current of the electrical users of the motor vehicle in all conditions, without the battery of the motor vehicle itself deteriorating rapidly, the alternator must distribute a relatively high current, even at the minimum speed of rotation of the drive shaft. In order to fulfil this need, in general it is not possible to increase beyond a certain limit the diameter of the driven pulley, and therefore the transmission ratio of the known transmission units, owing to the need to restrict the dimensions.
Thus, alternators with a large size are used, which distribute a sufficiently high current even at low speeds of rotation, but have the disadvantage firstly that they are costly and heavy, and secondly that their capacity is not generally used to the maximum for most of the running conditions of the vehicle.
The object of the present invention is to provide a drive unit for an alternator of a motor vehicle, which makes it possible to solve the above-described problems simply and economically, at the same time limiting the dissipation of energy at high speeds of rotation of the drive shaft, and which, in particular, has limited dimensions.
According to the present invention, a drive unit is provided for an alternator of a motor vehicle, the alternator comprising a drive shaft which can rotate around its own first axis; the said drive unit comprising a driven pulley which can rotate around its own second axis; and means for connection of the said pulley to the said drive shaft; characterised in that it additionally comprises a revolution-multiplier device and a torque-limiter device, which are in series with one another.
Preferably, the said revolution-multiplier and torque-limiter devices form part of the said means for connection.